The present application is based on Japanese priority application No. 2000-095895 filed on Mar. 30, 2000, the entire contents of which are hereby incorporated by reference.
The present invention generally relates to semiconductor devices and more particularly to a high-speed semiconductor triode having a compound-semiconductor channel layer.
Compound-semiconductor triodes, typical examples being a MESFET or a HEMT, is characterized by high operational speed due to high electron mobility of compound-semiconductor material used for the active layer thereof. Thus, such compound-semiconductor triodes are used extensively for high-frequency or ultra high-frequency applications including GHz band application.
In such compound-semiconductor triodes, too, there holds the scaling law, and efforts are made to reduce the gate length as much as possible for maximizing the operational speed.
A high-speed semiconductor triode having a short gate length is designed, in order to suppress the short-channel effect as much as possible, such that carriers are transported through a shallow, limited surface region of a compound-semiconductor layer used for the active layer of the semiconductor triode.
Thus, the quality of the crystal of the compound-semiconductor layer, particularly the quality of the surface part of the compound-semiconductor layer used for the active layer is extremely important for the operational characteristic of the semiconductor triode.
FIG. 1 shows the construction of a HEMT 10 according to a related art.
Referring to FIG. 1, the HEMT 10 is constructed on a semi-insulating InP substrate 11 and includes a channel layer 12 of undoped InGaAs formed epitaxially on the InP substrate 11 and an electron-supplying layer 13 of n-type InAlAs formed also epitaxially on the channel layer 12. A cap layer 14 of n+-type InGaAs is formed on the electron-supplying layer 13 epitaxially, and an opening 14A exposing the surface of the electron-supplying layer 13 is formed in the cap layer 14. Further, a gate electrode 15 Is formed on the exposed surface of the electron-supplying layer 13 in the opening 14A.
In the illustrated example, the gate electrode 15 is a so-called mushroom type or T-type electrode and includes a Ti layer 15A making a Schottky contact with the exposed electron-supplying layer 13, a Pt diffusion-barrier layer 15B formed on the Ti layer 15A, and a low-resistance Au electrode layer 15C having the mushroom-shape and formed on the Pt layer 15B.
By using the Au electrode 15C with such a mushroom-shape, it becomes possible to reduce the resistance of the gate electrode 15 while minimizing the gate-length of the gate electrode 15 simultaneously. The Pt diffusion barrier layer 15B, on the other hand, blocks the diffusion of Au atoms from the Au electrode into the electron-supplying layer 13. Further, the Ti layer 15A provided between the electron-supplying layer 13 and the Pt layer 15B improves the adherence of the Pt layer 15B to the electron-supplying layer 13.
In the HEMT 10 of FIG. 1, it should further be noted that ohmic electrodes 16 and 17 are formed on the InGaAs cap layer 14 in correspondence to contact regions 14B and 14C respectively. The ohmic electrode 16 constitutes a non-alloy ohmic electrode and includes a Ti layer 16A forming an ohmic contact with the n+-type cap layer 14, a Pt diffusion barrier layer 16B formed on the Ti layer 16A and a low-resistance Au electrode layer 16C formed on the Pt diffusion barrier layer 16B. The ohmic electrode 17 has a similar construction.
Further, the HEMT of FIG. 1 includes an SiN passivation film 18 covering the exposed part of the electron-supplying layer 13 and the contact regions 14B and 14C.
In such a conventional compound-semiconductor triodes, including also MESFETs in addition to HEMTs, the gate electrode 15 makes a direct contact with the semiconductor layer, and thus, there is a substantial risk that Ti atoms may cause a diffusion from the Ti adhesion layer 15A of the gate electrode 15 into the n-type electron-supplying layer 13 and further into the channel layer 12 underneath the electron-supplying layer 13. When such a diffusion of Ti is caused in the semiconductor layers constituting the channel of the triode, the threshold characteristic of the device is deteriorated seriously.
FIG. 2 shows such a change of the threshold voltage Vth for the case such a diffusion of Ti is caused from a gate electrode into a channel layer in the case of a conventional MESFET.
Referring to FIG. 2, it can be seen that the threshold voltage Vth increases generally linearly with the depth of penetration of the Ti atoms, and that the threshold voltage Vth changes as much as 0.1V with the penetration of only 1 nm in depth. Thus, there is a need for a structure, in compound-semiconductor triodes such as HEMTs or MESFETs, which is effective for suppressing the diffusion of TI atoms from the electrode into the compound-semiconductor layer.
Conventionally, it has been practiced in the art of compound-semiconductor Schottky diode to interpose a metal oxide layer between the Schottky electrode and the compound-semiconductor layer for suppressing the diffusion of metal elements from the Schottky electrode to the compound-semiconductor layer, and hence to suppress the change of Schottky barrier height. In relation to this, reference may be made to Japanese Laid-Open Patent Publication 4-69974.
In this prior art reference, the use of TiOx formed as a result of oxidation of the surface of the metallic Ti layer is described as an example of such a metal oxide layer.
FIG. 3 shows the effect of Ti diffusion on the Schottky barrier height xcfx86B of a Schottky diode.
Referring to FIG. 3, it can be seen that there occurs no substantial change of Schottky barrier height xcfx86B even when the Ti atoms have penetrated into the semiconductor layer with the thickness of several nanometers. Thus, it is concluded that, in the case of a semiconductor Schottky diode, the use of such a metal oxide layer between the semiconductor layer and the Schottky electrode causes no substantial change of diode characteristic.
In the case of a compound-semiconductor triodes such as a HEMT or a MESFET, on the other hand, the situation is different, and penetration Ti of only 1 nm depth in the channel region causes a serious change of the threshold voltage Vth.
In the fabrication process of a semiconductor triode, various annealing steps are applied after a Schottky electrode is formed on a channel layer as a gate electrode. Thus, the foregoing variation of the threshold voltage Vth, caused as a result of Ti penetration, remains a substantial problem in the art of compound-semiconductor triodes.
Accordingly, it is a general object of the present invention to provide a novel and useful compound-semiconductor triode wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a high-speed compound-semiconductor triode stable against thermal annealing process.
Another object of the preset invention is to provide a semiconductor triode, comprising:
a semiconductor layer including a channel layer;
a first ohmic electrode supplying carriers into said channel layer;
a second ohmic electrode collecting carriers from said channel layer; and
a gate electrode controlling a flow of said carriers through said channel layer from said first ohmic electrode to said second ohmic electrode,
said gate electrode including an insulating metal oxide film formed at an interface to a surface of said semiconductor layer.
According to the present invention, the threshold characteristic of the semiconductor triode is stabilized substantially by interposing the metal oxide film. Further, such a structure is advantageous for improving the yield of production of the device.
Preferably, the metal oxide film is formed of any of an oxide of a metal element selected from the group consisting of Ti, Co, Ni, Ta, Pr, Hf, Zr and Pd. The metal oxide film may be formed also at the interface between the first ohmic electrode and the semiconductor layer and the interface between the second ohmic electrode and the semiconductor layer. Preferably, the metal oxide film has a thickness allowing carrier tunneling therethrough. The metal oxide film may be provided so as to cover the surface of the semiconductor layer continuously from the first ohmic electrode to the gate electrode and from the gate electrode to the second ohmic electrode. The semiconductor triode of the present invention includes a HEMT and a MESFET.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.